For use in an exhaust emission control system having a DPF (Diesel Particulate Filter) and a PM sensor for detecting the amount of particulate matter (hereinafter may be referred to as the PM) in exhaust gas, a fault determination device for determining whether the PM sensor is faulty is conventionally known as disclosed, for instance, in JP-A-2010-275977. More specifically, a conventional technology applied to this fault determination device is used in a configuration in which the PM sensor is installed downstream of the DPF in an exhaust pipe, and determines, in accordance with a change in a PM sensor output prevailing after regeneration of the DPF, whether the PM sensor is faulty.
When the PM collection performance of the DPF is normal, the DPF can collect the PM in the exhaust gas. In other words, an extremely small amount of PM flows downstream of the DPF as far as the DPF's PM collection performance is normal. Even if an attempt is made to check whether the PM sensor generates a normal output in accordance with the arrival of the PM at the PM sensor, it is virtually difficult to perform such a check when an extremely small amount of PM flows downstream of the DPF.
As such being the case, the above conventional technology notes the fact that the PM collection performance is temporarily degraded during a certain period after the regeneration of the DPF, and determines whether the PM sensor is faulty by checking an output of the PM sensor during the period during which the PM collection performance is degraded. In other words, as a certain limit is imposed on the amount of PM collected by the DPF, it is necessary to recover the DPF's PM collection performance by regenerating the DPF as needed. According to the JP-A-2010-275977, the DPF's PM collection performance is temporarily degraded immediately after the above-mentioned DPF regeneration process is performed. While the PM collection performance is degraded, the amount of PM flowing downstream of the DPF is larger than while normal PM collection performance is exhibited. Whether or not the PM sensor is faulty can be determined by checking an output of the PM sensor during the period during which the PM collection performance is degraded.